This invention relates to electric boilers and more particularly to a conductivity control for such boilers.
Electric boilers generally include a plurality of electrodes which are at least partially submerged in a quantity of water contained within the boiler. The flow of electric current through the water and between the electrodes heats the water for the production of steam. As the water evaporates, the concentration of salts and other impurities in the water will tend to increase thereby affecting the water's conductivity. In order to achieve optimum operating conditions, it is therefore necessary to periodically replace at least a portion of the concentrated boiler water with fresh feed water. One prior art method of accomplishing this result was to withdraw and replace boiler water at preselected time intervals. This method was not always satisfactory because the boiler water replacement was made without regard to the concentration of impurities or boiler load.
Another prior art conductivity control system is disclosed in U.S. Pat. No. 3,760,775 wherein a current sensor determines when delivery of feed water is indicated and initiates a timer for discharging boiler water for a fixed time interval, after which a feed water valve is opened to permit the delivery of feed water. As the water level rises in the boiler, the resulting increase is current is detected by the current sensor terminating feed water delivery. This system was not wholly satisfactory, because the control was not able to adjust boiler water level in relation to the rapid changes in boiler pressure which accompany changes in steam loading changes.